Diameter droplet

D = mean droplet diameter (pm) s = surface tension (dyne- cm" ) d = density of liquid (g- cm" )... 

The aim of breaking up a thin film of liquid into an aerosol by a cross flow of gas has been developed with frits, which are essentially a means of supporting a film of liquid on a porous surface. As the liquid flows onto one surface of the frit (frequently made from glass), argon gas is forced through from the undersurface (Figure 19.16). Where the gas meets the liquid film, the latter is dispersed into an aerosol and is carried as usual toward the plasma flame. There have been several designs of frit nebulizers, but all work in a similar fashion. Mean droplet diameters are approximately 100 nm, and over 90% of the liquid sample can be transported to the flame. 

For example, at a frequency of 1 MHz, the effect of ultrasound in water of surface tension 73 dyn-cm and density 1 g-cm- is to produce longitudinal waves of about 12 pm. The resulting mean droplet diameter (D) is given by Equation 19.2. 

Aerosols (qv) are very finely divided sprays having droplet diameters of l ndash 30 p.m. They are used almost entirely as space sprays for appHcation to enclosures, particularly against flying insects. Aerosols are most conveniendy appHed by the familiar Hquefted gas dispersion or bomb but can be generated on a larger scale by rotary atomi2ers or twin duid atomi2ers. 

Median Diameter. The median droplet diameter is the diameter that divides the spray into two equal portions by number, length, surface area, or volume. Median diameters may be easily determined from cumulative distribution curves. 

Hollow Sprays. Most atomizers that impart swid to the Hquid tend to produce a cone-shaped hoUow spray. Although swid atomizers can produce varying degrees of hoUowness in the spray pattern, they aU seem to exhibit similar spray dynamic features. For example, detailed measurements made with simplex, duplex, dual-orifice, and pure airblast atomizers show similar dynamic stmctures in radial distributions of mean droplet diameter, velocity, and Hquid volume flux. Extensive studies have been made (30,31) on the spray dynamics associated with pressure swid atomizers. Based on these studies, some common features were observed. Test results obtained from a pressure swid atomizer spray could be used to iUustrate typical dynamic stmctures in hoUow sprays. The measurements were made using a phase Doppler spray analyzer. 

Droplet Size Corrections. The majority of correlations found in the Hterature deal with mean droplet diameters. A useflil equation for Sauter... 

Effect of Variables on Mean Droplet Size. Some of the principal variables affecting the mean droplet diameters for pressure swid atomizers may be expressed by equation 14. 

Droplet size, particularly at high velocities, is controlled primarily by the relative velocity between liquid and air and in part by fuel viscosity and density (7). Surface tension has a minor effect. Minimum droplet size is achieved when the nozzle is designed to provide maximum physical contact between air and fuel. Hence primary air is introduced within the nozzle to provide both swid and shearing forces. Vaporization time is characteristically related to the square of droplet diameter and is inversely proportional to pressure drop across the atomizer (7). 

Ue = liquid velocity relative to the gas, often approximately the terminal velocity of droplets (see Sec. 6 lor estimation) L/g = superficial gas velocity = droplet diameter... 

Nf = hqiiid-phase transfer units [Eq. (14-173)] k = hqiiid thermal conductivity p = hqiiid density Cp = hqiiid specific heat = droplet diameter t = time of contact... 

Pipeline Flow (Quenching) For the case of pipeline quenching, the flows are cociirrent. How closely the gas temperature approaches the liquid depends on where Ng varies with l/(droplet diameter)". 

Since the predicted droplet diameter at high velocity pipeline flow varies with (LVelocity) ", as shown by Eq. (14-201), the volumetric performance is strongly dependent on velocity ... 

Liquid-Column Breakup Because of increased pressure at points of reduced diameter, the liquid column is inherently unstable. As a result, it breaks into small drops with no external energy input. Ideally, it forms a series of uniform drops with the size of the drops set by the fastest-growing wave. This yields a dominant droplet diameter... 

Values of Pp and dp are droplet density, g/cm, and droplet diameter, cm Ig is the gas viscosity, P. All other terms were defined previously. Table 14-19 gives values of J calculated from experimental data of Jackson and Calvert. Values of J for most manufactured packing appear to fall in the range from 0.16 to 0.19. The low value of 0.03 for coke may be due to the porosity of the coke itself. 

This assumes a droplet diameter of 0.0005 ft. Fs is determined from ... 

When a liquid is dispersed into droplets the surface area is increased, which enhances the rates of heat and mass transfer. For a particular liquid dispersed at constant concentration in air the MIE varies with approximately the cube of surface average droplet diameter, hence the MIE decreases by a factor of about 8 when the surface average diameter D is halved (A-5-1.4.4). Ease of ignition is greatly enhanced for finely divided mists with D less than about 20 /rm, whose MIE approaches that of the vapor. Below 10 /rm a high flash point liquid mist (tetrahydronaphthalene) was found to behave like vapor while above about 40/rm the droplets tended to burn individually [ 142]. Since liquid mists must partially evaporate and mix with air before they ignite, the ease with which a liquid evaporates also affects MIE (Eigure 5-1.4.4). 

FIQURE 5-1.4.4. Schematic effect of droplet diameter on MIE of typical petroleum products. 

The model assumes that liquid evaporation is always the rate controlling step. At some point the model must fail, since as droplet size approaches zero the predicted MIE approaches zero rather than the MIE of the vapor in air. In practice, droplets having diameters less than 10-40 /rm completely evaporate ahead of the flame and burn as vapor (5-1.3). The model also predicts that the MIE continuously decreases as equivalence ratio is increased, although as discussed above, combustion around droplets is not restrained by the overall stoichiometry and naturally predominates at the stoichiometric concentration. It is recommended that the model be applied only to droplet diameters above about 20/rm and equivalence ratios less than about one. 

Samples must be introduced into the plasma in an easily vaporized and atomized form. Typically this requires liquid aerosols with droplet diameters less than 10 pm, solid particles 1-5 pm in diameter, or vapors. The sample introduction method strongly influences precision, detection limits, and the sample size required. 

Droplet diameter, when other data is not available ... 

Diameter for light phase, ft Diameter of particle, ft or equivalent diameter of spherical particle, ft Minimum diameter of particle that is completely collected, ft Diameter of particle, in. or mm Droplet diameter, ft... 

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